I work at a hotel, and just like a cruise ship, we want to be at 100% occupancy every night (or every sailing for a ship). There are strategies for this. As previously mentioned, selling a category guarantee instead of a cabin number is the best way. If you have 1200 cabins, but your outside verandahs are not selling, then open the system up to selling more inside or ocean view guarantees, and just upgrade as needed. Then there is the math. The person who paid the most and/or booked earliest is most likely to get upgraded first. If you paid $500 for a 5n cruise, but your friend Bob only paid $300, you're going to get upgraded so the cruise line can sell another inside for $300.
I think a good way to see if your inside guarantee will get upgraded ... goto the web and start looking at the category you want upgraded to. Does that sailing still have *a lot* of staterooms in that category left to sell? If they do, then there's a better chance of you getting upgraded. Are there only 5 ocean views left to book on that ship? Well if you want to see the sun in the morning, you better pay for the ocean view.
Its also a known fact that hotels will overbook a certain room category, usually the cheapest one, knowing they can upgrade guests when the time comes. Why? Again 100% occupancy is the goal. A suite that is empty tonight makes no money. A suite that has someone that originally booked a $99/night room is still making some money, and then the next guest in line stays in the $99/night room. The hotel still sells out. Guest A feels great because they got the upgrade, Guest B is okay because they got what they paid for.
If you're a super bargain hunter and book as cheap as it can go, will you get an upgrade? I won't say no, but I will say the odds are slim.
Disney Dream - Dec 2012 4n Bahamas
Carnival Imagination - Dec 2011 4n Western
Carnival Imagination - Nov 2010 4n Western
Costa Atlantica - Dec 2009 7n Eastern
My opinions are mine and mine alone. They do not represent the opinions of The Walt Disney Company nor any of its subsidiaries.